Re: [PATCH v3 2/6] ext4: move inode extension/truncate code out from ext4_iomap_end()

Matthew Bobrowski
2019-09-24T09:50:53Z

On Mon, Sep 23, 2019 at 06:21:15PM +0200, Jan Kara wrote:
> On Thu 12-09-19 21:04:00, Matthew Bobrowski wrote:
> > @@ -233,12 +234,90 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
> > return iov_iter_count(from);
> > }
> >
> > +static int ext4_handle_inode_extension(struct inode *inode, loff_t offset,
> > + ssize_t len, size_t count)
>
> Traditionally, we call one of the length arguments 'copied' or 'written' to
> denote actual amount of data processed and the original length is called
> 'len' or 'length' in iomap code. Can you please rename the arguments to
> follow this convention?

Sure, I will go with 'written'.

> > +/*
> > + * The inode may have been placed onto the orphan list or has had
> > + * blocks allocated beyond EOF as a result of an extension. We need to
> > + * ensure that any necessary cleanup routines are performed if the
> > + * error path has been taken for a write.
> > + */
> > +static int ext4_handle_failed_inode_extension(struct inode *inode, loff_t size)
> > +{
> > + handle_t *handle;
> > +
> > + if (size > i_size_read(inode))
> > + ext4_truncate_failed_write(inode);
> > +
> > + if (!list_empty(&EXT4_I(inode)->i_orphan)) {
> > + handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
> > + if (IS_ERR(handle)) {
> > + if (inode->i_nlink)
> > + ext4_orphan_del(NULL, inode);
> > + return PTR_ERR(handle);
> > + }
> > + if (inode->i_nlink)
> > + ext4_orphan_del(handle, inode);
> > + ext4_journal_stop(handle);
> > + }
> > + return 0;
> > +}
> > +
>
> After some thought, I'd just drop this function and fold the functionality
> into ext4_handle_inode_extension() by making it accept negative 'len'
> argument indicating error. The code just happens to be the simplest in that
> case (see below).
>
> > @@ -255,7 +334,18 @@ ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
> > if (ret)
> > goto out;
> >
> > + offset = iocb->ki_pos;
> > ret = dax_iomap_rw(iocb, from, &ext4_iomap_ops);
> > + if (ret > 0 && iocb->ki_pos > i_size_read(inode))
> > + error = ext4_handle_inode_extension(inode, offset, ret,
> > + iov_iter_count(from));
>
> You need to sample iov_iter_count(from) before calling dax_iomap_rw(). At
> this point iov_iter_count(from) is just what's left in the iter after
> writing what we could.
>
> Also I don't think the condition iocb->ki_pos > i_size_read(inode) is
> correct here. Because it may happen that offset + count > i_size so we
> allocate some blocks beyond i_size but then we manage to copy only less so
> offset + ret == iocb->ki_pos <= i_size and you will not call
> ext4_handle_inode_extension() to truncate allocated blocks beyond i_size.

Yeah, this makes sense. I will implement it this way. Once again, appreciate
your suggestions.

> So I'd just call ext4_handle_inode_extension() unconditionally like:
>
> error = ext4_handle_inode_extension(inode, offset, ret, len);
>
> and have a quick check at the beginning of that function to avoid starting
> transaction when there isn't anything to do. Something like:
>
> /*
> * DIO and DAX writes get exclusion from truncate (i_rwsem) and
> * page writeback (i_rwsem and flushing all dirty pages).
> */
> WARN_ON_ONCE(i_size_read(inode) != EXT4_I(inode)->i_disksize);
> if (offset + count <= i_size_read(inode))
> return 0;
> if (len < 0)
> goto truncate;
>
> ... do the heavylifting with transaction start, inode size update,
> and orphan handling...
>
> if (truncate) {
> truncate:
> ext4_truncate_failed_write(inode);
> orphan_del:
> /*
> * If the truncate operation failed early the inode
> * may still be on the orphan list. In that case, we
> * need try remove the inode from the linked list in
> * memory.
> */
> if (inode->i_nlink)
> ext4_orphan_del(NULL, inode);
> }

--<M>--